The present invention generally relates to the manufacture of thermoplastic bags and, more particularly, to a process and apparatus for cutting and sealing multiple plies of thermoplastic material in the manufacture of articles, such as bags, having thickened sections, such as zipper closures.
For many years, devices for concurrently cutting and sealing multiple plies of thermoplastic material (hereinafter referred to as cutter/sealer devices) have been used to manufacture plastic trash bags, folded sandwich bags and other non-zippered bags from uniform film stock. U.S. Pat. No. 3,033,257 to Weber discloses an example of one such prior art device. Generally speaking, quality seals can be produced at high cycle rates by a cutter/sealer device due to the uniformity of thickness of the film stock used.
More recently, cutter/sealer devices have been employed in the manufacture of bags from multiple plies of thermoplastic film of non-uniform thicknesses. U.S. Patents to Howard (U.S. Pat. No. 3,986,914), Boccia (U.S. Pat. No. 4,024,010), Siegel (U.S. Pat. No. 4,304,615), Tumminia (U.S. Pat. No 4,396,449) and Savicki (U.S. Pat. No. 4,658,433) disclose examples of such prior art devices. The non-uniformity of film thickness is due to the provision of integral and nonintegral zipper closures thereon which have substantially greater thickness than the remainder of the film, for example, a 70 mil thick zipper closure provided on a 2 mil thick film.
Processing of non-uniform thickness or zippered film stock into plastic bags introduces several problems which can lead to an increase in leaks along the sealed longitudinal side edges of the bag at the zipper closure end seals as well as at the remaining longitudinal side edge seals. One problem relates to the manner in which the cutter/sealer device has to be operated to cut and seal the thicker zipper closure area. The thicker zipper closure requires the device to be operated at higher temperatures and lower cycle speeds in order to cut through the zipper closure. These operating conditions will tend to produce rougher and more distorted seals along the remainder of the longitudinal side edges of the bag. These distorted areas are often potential sites of leaks in the plastic bag.
Another problem relates to stretching of the film caused by winding over the thicker profile created by the presence of the zipper closure on the film. Since the zippered film stock is wound under tension on mill rolls, it typically stretches due to being wound on top of the thicker profile. Stretching of the film promotes the occurrence of additional problems, such as film wrinkling, entrapping of air, and inadequate film creasing during folding, all of which provide additional potential sites for leaks.
Attempts have been made in the prior art to address the problem of processing zippered film stock to provide a bag having leak proof seals. Such attempts have pretreated the film stock using a combination of heat and pressure to smash the zippered portion of the film stock to reduce the thickness presented to the hot knife to sever and seal through. Heat has been produced either thermally or through the use of ultrasonic vibrations. However, while such attempts have improved somewhat the integrity of the side weld seals at the zipper closure portions of the bag, the processes used have resulted in leaks appearing elsewhere in the bag and through the zipper closure which results from distortion in the zipper closure introduced by the application of pressure during preheating.
Consequently, a need exists for improvements which will better adapt a cutter/sealer device for use in manufacture of plastic bags from non-uniform thickness film stock, primarily film having thickened sections due to the presence of zipper closures.